Inverters have become installed in many types of electrical equipments, as efficient variable-speed control units. Inverters are switched at a frequency of several kHz to several ten kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, and consequently, to apply a voltage twice as high as the inverter output voltage at the maximum. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT, is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and voltage decay due to the connection cable is also low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electrical equipments, such as inverter-related equipments, for example, high-speed switching devices, inverter-driven rotary electric machines, and transformers, use is made of insulated wires, which are mainly enameled wires, as magnet wires in the coils. Further, as described above, since a voltage almost twice as high as the inverter output voltage is applied in inverter-related equipments, it has become required to minimize the inverter surge deterioration of the enameled wire, which is one of the materials constituting the coils of those electrical equipments.
In general, partial discharge deterioration is a phenomenon in which an electrical-insulation material undergoes, in a complicated manner, for example, molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge of the insulating material, sputtering deterioration, thermal fusion or thermal decomposition deterioration caused by local temperature rise, and chemical deterioration caused by ozone generated due to discharge. For this reason, reduction in thickness, for example, is observed in the actual electrical-insulation materials, which have been deteriorated as a result of partial discharge.
It is believed that inverter surge deterioration of an insulated wire also proceeds by the same mechanism as in the case of general partial discharge deterioration. That is, inverter surge deterioration of an enameled wire is a phenomenon in which partial discharge occurs in the insulated wire due to the surge voltage with a high peak value, which is occurred at the inverter, and the coating of the insulated wire causes partial discharge deterioration as a result of the partial discharge; in other words, the inverter surge deterioration of an enameled wire is high-frequency partial discharge deterioration.
The recent electrical equipments require insulated wires, which are capable of withstanding a surge voltage of 500 V. That is, there is a demand for insulated wires that have a partial discharge-occurring voltage of 500 V or more. Herein, the partial discharge-occurring voltage is a value that is measured by a commercially available apparatus called partial discharge tester. Measurement temperature, frequency of the alternating current voltage to be used, measurement sensitivity, and the like are values that may vary as necessary, but the above-mentioned value is an effective value of the voltage at which partial discharge occurs, which is measured at 25° C., 50 Hz, and 10 pC.
When the partial discharge-occurring voltage is measured, a method is used in which the most severe condition possible in the case where the insulated wire is used as a magnet wire is envisaged, and a specimen shape is formed which can be observed in between two closely contacting insulated wires. For example, in the case of an insulated wire having a circular cross-section, two insulated wires are brought into linear contact by spiral twisting the wires together, and a voltage is applied between the two insulated wires. Alternatively, in the case of an insulated wire having a rectangular cross-section, use is made of a method of bringing two insulated wires into planar contact through the planes, which are the long sides of the insulated wires, and applying a voltage between the two insulated wires.
In order to obtain an insulated wire that does not cause partial discharge, that is, having a high partial discharge-occurring voltage, so as to prevent the deterioration of the enamel layer of the insulated wire caused by the partial discharge, it is thought to utilize a method of using a resin low in specific permittivity (dielectric constant) in the enamel layer or increasing the thickness of the enamel layer. However, the resins of commonly used resin varnishes generally have a specific permittivity between 3 and 4, and none of the resins have particularly low specific permittivity. Further, upon considering other properties (heat resistance, solvent resistance, flexibility, and the like) required from the enamel layer, it is not necessarily possible in reality to select a resin low in specific permittivity. Therefore, in order to obtain a high partial discharge-occurring voltage, it is indispensable to increase the thickness of the enamel layer. When the resins having a specific permittivity of 3 to 4 are used in the enamel layer, if it is intended to obtain a targeted partial discharge-occurring voltage of 500 V or higher, it is necessary based on the experience to set the thickness of the enamel layer at 60 μm or more.
However, in order to thicken the enamel layer, the number of passages made through the baking furnace in the production process is increased, the thickness of a coating formed from copper oxide at the surface of copper, which is a conductor, is increased, and thereby the adhesive force between the conductor and the enamel layer is lowered. Particularly, in the case of obtaining an enamel layer with thickness 50 μm or more, the number of passages made through the baking furnace exceeds 10 times. It has been known that if this number of passages exceeds 10 times, the adhesive force between the conductor and the enamel layer is conspicuously lowered.
It is also thought to utilize a method of increasing the thickness that can be formed by a single baking step, in order not to increase the number of passages made through the baking furnace. However, this method has a drawback that the solvent of the varnish is not completely vaporized and remains in the enamel layer as voids.
Attempts have been made hitherto to impart added values in terms of properties (properties other than the partial discharge-occurring voltage) to the enameled wire by providing a resin coating at the outer surface of the enameled wire. For example, Patent Literatures 1 to 3 are mentioned as techniques of the related art in terms of the constitution of providing an extrusion-coated layer on an enamel layer. However, these techniques were not so satisfactory in terms of the constitution of the thickness of the enamel layer or the extruded coating, from the standpoint of balancing between the partial discharge-occurring voltage and the adhesiveness between the conductor and the enamel layer.
Further, it has become demanded to further improve various performances, such as heat resistance, mechanical properties, chemical properties, electrical properties, and reliability, in the electrical equipments developed in recent years, as compared to the conventional electrical equipments. Under the situations, excellent abrasion resistance, thermal aging resistance, and solvent resistance have become required from insulated wires, such as enameled wires, that are used as magnet wires for electrical equipments for aerospace use, electrical equipments for aircraft, electrical equipments for nuclear power, electrical equipments for energy, and electrical equipments for automobiles.
Further, electrical equipments, which are represented by transformers or rotary electric machines, such as motors, have recently advanced in size reduction and performance improvement, and it becomes found that, in many applications, insulated wires to be used are pushed into a quite narrow space to pack. Specifically, it is not an exaggeration to say that the performance of a rotary electric machine depends on how many electrical wires can be placed and packed in a core slot in the rotary electric machine. As a result, the ratio of the cross-sectional area of the conductor to the cross-sectional area of the core slot (packing factor) is significantly increasing in recent years.
When electrical wires each having a circular cross-section are closely packed at the inside of a core slot, the space serving as dead space and the cross-sectional area of the respective insulation coating become important factors. For this reason, users attempt to increase the packing factor as much as possible, by press-fitting more electrical wires into a core slot, as far as the electrical wire having a circular cross-section is deformed. However, since reducing the cross-sectional area of the insulation coating sacrifices electrical performance thereof (insulation breakdown or the like), such reduction has not been carried out.
For the reasons discussed above, it has been lately attempted to use a rectangular wire in which the conductor has a shape similar to a quadrilateral (square or rectangle), as a means for increasing the packing factor. Use of a rectangular wire exhibits a dramatic effect in increasing the packing factor. However, since it is difficult to uniformly apply an insulation coating on a rectangular conductor, and since it is particularly difficult to control the thickness of the insulation coating in an insulated wire having a small cross-sectional area, the use of a rectangular wire is not so widely spread.
Working resistance of a coating is a property of an insulation coating required when coil-winding for a motor rotary electric machine or a transformer. This is because when the coating of an electrical wire is damaged upon the coil working step described above, the electrical insulation performance deteriorates.
Various methods have been conceived as the method of imparting this working resistance to the coating of an electrical wire. Examples thereof include a method of reducing surface damage at the time of working into a coil, by imparting a lubricating property to the coating, and thereby lowering the coefficient of friction; and a method of retaining the electrical insulation performance, by improving the adhesiveness between the coating and the electrical conductor, and thereby preventing the coating from being peeled off from the conductor.
As the former method of imparting lubricating performance, use has been traditionally employed of a method of applying a lubricant, such as wax, on the surface of an electrical wire; or a method of imparting lubricating performance, by adding a lubricant to the insulation coating, and making the lubricant to bleed out to the surface of the electrical wire at the time of producing the electrical wire. There are many examples of the former method. However, since the method of imparting this lubricating performance does not enhance the strength of the coating of the electrical wire itself, the method seems to be effective against the surface damage factors, but there has been in fact limitation on the effect of the method.
First, in regard to the above-mentioned method of reducing the coefficient of friction at the surface of the insulation coating, as a means that has been conventionally used, for example, Patent Literature 4 or the like proposes a method of applying wax, oil, a surfactant, a solid lubricant, or the like on the surface of an insulated wire; Patent Literature 5 or the like proposes a method of applying, to an insulated wire, a friction reducing agent formed from a wax capable of being emulsified in water and a resin capable of being emulsified in water and solidifying when heated, and baking the insulated wire before use; and Patent Literature 6 or the like proposes a method of inducing lubrication by adding a fine powder of polyethylene to the insulation coating material itself. The above methods have been conceived so as to enhance the surface lubricating property of the insulated wire, and to consequently protect the insulation layer from surface damage through surface sliding of an electrical wire.
However, since these methods of adding a fine powder are complicated in the technique of adding the fine powder, and dispersing is difficult, a method of adding such a fine powder in the form of being dispersed in a solvent, into an insulation coating material, is employed in many cases.
These self-lubricating components can have an improvement of the self-lubricating performance (coefficient of friction) by the lubricating components, but do not have an improvement of properties, such as reciprocating abrasion caused by working resistance. Furthermore, many types of self-lubricating components, such as polyethylene and poly(tetrafluoroethylene), become separated off in the insulation coating material, due to the difference in the specific gravity between the insulation coating material and the self-lubricating components, it is necessary to pay careful attention upon using these coating materials.
As a means for solving these problems, a means described in Patent Literature 7 is proposed, and inventions of the following respective constitution are described as the means.
(1) An insulated wire, having at least one enamel baked layer around the outer periphery of a conductor, and at least one extrusion-coated resin layer on the outer side thereof, wherein the sum of the thickness of the enamel baked layer and the extrusion-coated resin layer is 60 μm or more;(2) The insulated wire described in item (1), wherein the thickness of the enamel baked layer is 50 μm or less;(3) The insulated wire described in item (1) or (2), wherein the extrusion-coated resin layer is formed from a resin material having a tensile modulus of elasticity at 25° C. of 1,000 MPa or more and a tensile modulus of elasticity at 250° C. of 10 MPa or more;(4) The insulated wire described in any one of items (1) to (3), having the at least one enamel baked layer around the outer periphery of the conductor having a rectangular cross-section, and the at least one extrusion-coated resin layer on the outer side thereof, wherein the thickness of the extrusion-coated resin layer provided on one pair of two sides facing each other of the cross-section is different from the thickness of the extrusion-coated resin layer provided on the other pair of two sides facing each other;(5) The insulated wire described in any one of items (1) to (4), having an adhesive layer between the enamel baked layer and the extrusion-coated resin layer, wherein the adhesive force between the enamel baked layer and the extrusion-coated resin layer is reinforced, by using the adhesive layer as a medium; and(6) A method of producing the insulated wire described in item (5), comprising: baking a varnished resin, around the outer periphery of the enamel baked layer, to form the adhesive layer; and then, bringing the adhesive layer into contact with the extrusion-coated resin, which is in a molten state at a temperature higher than the glass transition temperature of the varnished resin, thereby to cause thermal fusion of the enamel layer and the extrusion-coated resin layer.
Patent Literature 7 describes the following matters, as objects of their invention. The above invention is intended to provide an insulated wire high in partial discharge-occurring voltage. The above invention is also intended to provide an insulated wire that can realize thickening of the insulating layer for increasing the partial discharge-occurring voltage, without lowering the adhesive strength between the enamel layer and the conductor of the insulated wire. Further, the above invention is also intended to provide an insulated wire which satisfies the requirements of abrasion resistance, thermal-aging resistance, and solvent resistance, which are required from an insulated wire. The above invention is also intended to provide an insulated wire capable of having an increased packing factor, without lowering the partial discharge-occurring voltage. Moreover, the above invention is also intended to provide an insulated wire having satisfactory insertability at the time of working into a coil of a rotary electric machine, such as a motor. The above invention is also intended to provide an insulated wire capable of preventing a lowering in the partial discharge-occurring voltage at a bent portion even in the case of performing bending at a small radius, and to provide a method of producing the insulated wire.
The following are mentioned in Patent Literature 7, as the effects of their invention.
That is, the insulated wire of the above invention satisfies both of the “partial discharge-occurring voltage” and the “adhesive strength between the conductor and the enamel layer,” and suppresses the occurrence of inverter surge deterioration.
Further, when the thickness of the enamel layer is set at 50 μm or less, the number of passages made through the baking furnace can be reduced, and thus the adhesive force between the conductor and the enamel layer can be prevented from being extremely lowered.
Furthermore, when the extrusion-coated resin layer is formed from a resin material having a tensile modulus of elasticity at 25° C. of 1,000 MPa or more and having a tensile modulus of elasticity at 250° C. of 10 MPa or more, the insulated wire is also excellent in abrasion resistance, thermal-aging resistance, and solvent resistance.
Furthermore, in the case of an insulated wire with a conductor having a rectangular cross-section, when the thickness of the extrusion-coated resin layer formed on one pair of planes where discharge occurs is within a predetermined thickness, the partial discharge-occurring voltage can be maintained even if the thickness of the extrusion-coated resin layer formed on the other pair of planes facing each other is smaller than the predetermined thickness, and further the packing factor can be increased.
The insulated wire of the above invention has a small coefficient of static friction, and has favorable insertability in the case of being worked into a coil of a rotary electric machine.
Moreover, when the adhesive strength is enhanced by introducing a layer having an adhesive function between the enamel layer and the extrusion-coated resin layer, occurrence of wrinkles such as described above can be prevented.
In addition, the insulated wire of the above invention can be favorably produced, by baking a varnished resin around the outer periphery of the enamel baked layer, to form an adhesive layer, and then bringing the adhesive layer into contact with the extrusion-coated resin which is in a molten state at a temperature higher than the glass-transition temperature of the resin utilized in the adhesive layer, and thereby thermally fusing the enamel baked layer with the extrusion-coated resin layer.
However, the following new technical problems have been raised in recent years.
(i) Insulation Performance Maintainability at Worked Portion (Working Resistance)
When a magnet wire is worked into a rotary electric machine, such as a motor, various stresses are applied. Above all, in a step referred to as bending, the bending of a wire is conducted, using a front jig as a fulcrum. In particular, in the case of wires having conductors with large diameters, or in the case of wires having insulation coatings with large thickness, each of which are increasingly used in recent years, stress is increased as much, and the force applied to the front jig and the fulcrum for pressing to the wire is also increased. In this case, at the portion of the wire contacting with the front jig, a compression mark remains on the insulation coating of the wire, and the thickness of the insulation layer is locally decreased. On the outer side of the bent portion R, the coating is stretched so that the insulation thickness is decreased. As a result, there is a problem that the electrical insulation property is lowered at these portions.
(ii) Coating Shape Maintainability at Worked Portion (Working Resistance)
After the bending of the wire such as described above, the strain exerted to the insulation coating of the wire at the time of the bending, is not negligible. In particular, in recent years, there are problems that the form of working provided on the wire is becoming more severe for the reason of size reduction and efficiency increasing of rotary electric machines; and that the strain is increased, which is locally applied on the insulation coating, for example, in a wire with a conductor having a large diameter or a wire with an insulation coating having a large thickness such as described above, which may cause breakage of the insulation coating after the working. In particular, after a heat cycle is performed after the working, the superiority or inferiority thereof appears significantly.
(iii) Insulation Performance Maintainability after Thermal Aging (Heat Resistance)
In various fields where rotary electric machines are used, there are many instances, for example, in which the voltage to be applied becomes higher due to an increase in the efficiency of the rotary electric machine, or in which the heat emission property cannot be sufficiently secured due to size reduction. Recently, the demand for the heat resistance of a rotary electric machine, that is, similarly the demand for the heat resistance of a magnet wire, is also rising. In particular, even after a coating is instantaneously and intermittently exposed to a high temperature above the designed temperature, a sufficient insulation performance is required of the coating.
It has become known that further studies are necessary against those technical problems, in addition to the countermeasures proposed in Patent Literature 7.    Patent Literature 1: JP-A-59-040409 (“JP-A” means unexamined published Japanese patent application)    Patent Literature 2: Japanese Patent No. 1998680 (JP-B-7-031944 (“JP-B” means examined Japanese patent publication))    Patent Literature 3: JP-A-63-195913    Patent Literature 4: JP-A-61-269808    Patent Literature 5: JP-A-62-200605    Patent Literature 6: JP-A-63-29412    Patent Literature 7: JP-A-2005-203334